Ethernet cables are used largely for networks and broadband, and are useful for connecting devices to the internet where you're not using a wireless connection.
First of all to set up your own ethernet network, you will need to choose your cable category (referred to in short as'Cat') which will either be Cat 5e, Cat 6 or higher.
You can buy Ethernet cables at most computer shops or you can have it supplied by your cable installers.
The category cable you choose which will affect bandwidth, as the category determines how many signals can pass through the cable at once.
For gigabit operation (1000BaseT) you will require at least Cat 5e, and for higher you will need Cat 6 or Cat 6a which can be considered as 'future proofing' as this will last you longer before becoming obsolete like Cat 5.
If you currently have Cat 5 or lower installed for your network cabling and wish to have this upgraded to a Cat 5e or Cat 6 system (it is recommended that you go for the latter which can be considered an investment), then you will probably need a professional data cabling installation company to replace these cables.